The present invention relates to a method of arranging control channels for exchanging control signals between a base station and a mobile station in mobile communications.
Control channels shared by a plurality of mobile stations in a conventional mobile communication system are mainly classified into paging and access control channels. In a mobile station access mode, a mobile station paging mode, or a mobile station location registration mode, control signals are transmitted between a base station and a mobile station through these control channels. In a conventional system, the paging and access control channels are arranged as independent channels.
The term "radio control channel" in this specification represents a physical radio control channel. For example, this channel is defined as a radio channel having a given frequency in FDMA, or as a radio channel constituted by a frequency and time slots in TDMA. The term "control channels" include functionally classified paging and access control channels in a broad sense.
The radio zone configuration as an assumption of a control channel arrangement will be described below.
FIG. 7 is a view for explaining a radio zone configuration and a paging area.
One radio zone 51 (a hexagonal portion in FIG. 7) is formed to have one base station as its center. A paging area 52 (a portion surrounded by a thick line in FIG. 7) is formed to perform location registration and paging of a plurality of radio zones 51. Control channels are arranged under the zone configuration, as will be described with reference to FIG. 8.
A P channel (paging channel or PCH) is a paging control channel which is commonly used within the location registration area and the paging area. A downward P channel 53 serves as a control channel for commonly transmitting paging information to mobile stations in radio zones within the paging area.
Common information of radio zones, and the like are also broadcast through the downward P channel 53. On the downward P channel, transmitting timings of the base stations in the same paging area are synchronized each other with a given frequency, and the base stations transmit the same signals simultaneously to mobile stations within the paging area.
An upward P channel 54 serves as a channel for transmitting to the base station a paging response signal in response to an paging signal to a mobile station. The upward P channel 54 controls a transmission timing of the mobile station so as to prevent signal collision on the basis of control from the base station and transmits the paging response signal. The P channel is called a broad control channel in a sense that the same information is transmitted to the plurality of radio zones.
An A channel (access channel or ACH) is a access control channel used for causing a mobile station to transmit information. The A channels are independent of each other in units of radio zones. An upward A channel 56 is a random access channel. The mobile station transmits an access signal to the base station, and the base station transmits response signals to the mobile station through corresponding downward A channels 55. The A channel also transmits a location registration signal, a communication channel designation signal required to establish a communication channel. The A channel is called a narrow control channel in a sense that different pieces of information are transmitted in units of radio zones.
In mobile communications, a so-called traffic taper in which traffic densities are different in units of radio zones typically occurs in an urban area. Therefore, it is important to arrange control channels under the above circumstances.
A conventional control channel arrangement is shown in FIGS. 9(a) to 9(c). FIG. 9(a) is a view showing a relationship between radio zones and a traffic taper, FIG. 9(b) is a view showing a control channel arrangement in a radio zone having a higher traffic density, and FIG. 9(c) is a view showing a control channel arrangement in a radio zone having a lower traffic density.
The number of P channels respectively assigned to the radio zones is determined in accordance with the number of subscribers in the entire paging area. The P channels are equally arranged in all the radio zones.
Since simultaneous transmitting by plural base stations is performed, the P channels must have identical control channel structures in all the radio zones (PCH 1 and PCH 2 in FIGS. 9(b) and 9(c)).
The required number of ACHs is determined in accordance with the number of subscribers in each radio zone. That is, the ACHs are proportionally assigned to the radio zones in accordance with the numbers of subscribers in the radio zones.
More specifically, three A channels (ACH 1, ACH 2 and ACH 3) are used in a radio zone 51a having a high traffic density, while one one A channel (ACH 1) is used in a radio zone 51b having a low traffic density.
In a conventional system, since a paging response signal which is returned from one radio zone within a paging area is transmitted through the P channel serving as a broad channel, the upward P channel is not used during transmission of the response signal in the radio zones having no mobile stations which respond to the paging. As a result, efficiency of use of the upward P channel is degraded, resulting in inconvenience.
Assume that a traffic taper is present. Since simultaneous transmitting is performed within a paging area, the same number of P channels as that in a radio zone having a high traffic density must be arranged in a radio zone having a low traffic density. The traffic zone having a low traffic density has poor efficiency of use of the upward P channel as compared with the radio zone having a high traffic density, thus further degrading efficiency as a whole. At least one P channel and at least one A channel must be arranged in each radio zone. Efficient utilization of frequency resources cannot be achieved, and thus the number of control transmitters/receivers is increased.
Since the P and A channels are physically independent radio channels, channel switching is required to change the P channel to the A channel, and vice versa, thus requiring complicated control.
When the P and A channel numbers are to be changed in accordance with a change in zone arrangement, the numbers of control transmitters/receivers in the base stations, must be increased/decreased, respectively.
In a system such as an AMPS system wherein simultaneous transmission of plural radio zones is performed using different radio control channel frequencies in units of radio zones in the paging area, in a period such as an initial period during which a traffic density is low, or in an area having a low traffic density in a rural area, an A-channel signal can also be transferred through a P-channel radio control channel.
However, when the number of required radio control channels is two or more, other independent A channels are arranged.
In general, the P channel is used to transfer information (paging) to all mobile stations in a broad paging area, and efficiency per zone is as low as 1/(zone number in the paging area). The upward A channel is a random access channel, and its efficiency is generally low, i.e., a maximum of about 10%. The independent channels ar arranged due to independent upward and downward channels having poor efficiency. Therefore, utilization efficiency of frequency resources and transmitters/receivers is poor.